This invention relates to a refrigeration system employing a motor driven centrifugal compressor having a diffuser with a movable wall and, in particular, to controlling the positioning of the movable wall to continually maintain the compressor at or about optimum operating conditions as the load thereon changes.
Variable speed centrifugal compressors have been used in the art to control the amount of refrigerant flowing through heating and cooling systems using the carnot cycle. These variable speed machines, although working quite well in practice, are relatively expensive to build and oftentimes difficult to maintain. Accordingly, these machines have not found wide spread use in the industry. Most centrifugal compressors that are presently in use are arranged to turn at a fixed operational speed and control over the machinery is achieved by adjusting the positioning of guide vanes located at the entrance to the impeller wheel.
In a refrigeration system utilizing a fixed speed centrifugal compressor, the mass rate of flow delivered to the compressor impeller is typically varied to meet the changing demands placed on this system. Under maximum flow conditions, the amount of refrigerant leaving the impeller blades is sometimes more than the diffuser section can handle and the flow thus becomes choked at the diffuser entrance or throat. At relatively low flow rates, the refrigerant flow moving through the diffuser section becomes unstable and a partial reversal in the flow pattern takes place. This, in turn, produces a good deal of noise and a dramatic reduction in machine efficiency. Ultimately, a complete reversal in the flow pattern is experienced whereupon the compressor will stall or surge. The flow rate spread between a choked condition and a surge condition generally defines the operating range of the machine. In a fixed speed compressor, wherein the inlet guide vanes are used to maintain flow control, this operating range is typically very narrow.
In a copending U.S. patent application Ser. No. 531,019, filed Sept. 12, 1983 in the name of Kirtland, there is described a centrifugal compressor having a diffuser in which one wall is movably mounted so that the width of the diffuser opening can be varied to meet different load conditions placed on the system. In a later filed U.S. Pat. No. 4,503,684, which issued to Mount et al, there is described a control system for use in conjunction with a variable width diffuser for monitoring the lift and the load placed on the compressor and adjusting the movable wall position to hold the machine at or close to optimum operating conditions. The load is determined by measuring the current flow through the compressor motor while the lift is determined by comparing the temperatures of the water leaving the evaporator and the condenser of the refrigeration system.
The Mount et al control system works exceedingly well in practice but requires the use of a minicomputer to track both load and lift. The present invention recognizes the many advantages of the Mount et al control system and provides a method by which the diffuser width can be varied using less equipment thereby reducing the cost of the refrigeration system without significantly sacrificing operating efficiency.